In the prior art, the processing of expandable polystyrene beads has consistently included an intermediate step of steam pre-expansion. Polystyrene beads containing about six percent of pentane gas at a bulk density of 40 pcf is metered into a large cylindrical tank with live steam. The steam permeates the beads, softening them and vaporizing the pentane to cause expansion of the beads usually in the density range of about 1 to 5 pcf. The freshly expanded material contains pentane vapor and condensed steam. As the material cools, a negative pressure is created in the cells of the beads. The surface of the beads contains moisture and the beads must be delivered to storage bins to allow the surface moisture to evaporate and the excess pentane to escape while air permeates the material to create molding stability. This storage process usually requires twenty-four hours.
The resulting aged and matured material known in the art as prepuff is now ready for molding. Molding is customarily done in a large block mold eight or sixteen feet long by fifty inches in height, by about twenty inches thick. The prepuff is metered into the mold cavity by gravity until the cavity is filled. The large molds have steam chambers on both sides. The steam enters through vents in the side walls softening the prepuff and causing it to expand. Because the material is confined in the mold cavity, it can be fused into an integral mass with heat and pressure. However, if too much steam enters the mold cavity, severe shrinkage, long cycles and trapped moisture in the material results. Also a very large steam source is required for the process. After molding, approximately twenty-four hours curing time is required to allow complete shrinkage and stabilization of the molded product. If a relatively large amount of water is trapped in the product during molding, severe cutting problems result.
Conventional block molds sold for this operation are made oversize to allow for shrinkage and collapse. Normally, twelve to fifteen percent of the material is lost in the process. Additionally, because of the thickness of the large block molds, there exists a great density gradient from the middle of the board to its outside skins, causing variations in the physical properties from one cut board to another.
The conventional method, as described, also produces a product which, if ignited, will show pentane flashes for at least a week after molding. In the present invention, a new process has been devised which reduces power consumption, shortens cycles, and saves on material and manpower.
Examples of the patented prior art are contained in U.S. Pat. Nos. 3,577,360 and 4,032,609.